Geological Survey Bulletin 1347The Geologic Story of Yellowstone National Park

HOT WATER AND STREAM PHENOMENA

Although Yellowstone is geologically outstanding in
many ways, the great abundance, diversity, and spectacular nature of its
thermal (hot-water and steam) features were undoubtedly the primary
reasons for its being set aside as our first National Park (fig. 43).
The unusual concentration of geysers, hot springs, mudpots, and
fumaroles provides that special drawing card which has, for the past
century, made the Park one of the world's foremost natural
attractions.

To count all the individual thermal features in
Yellowstone would be virtually impossible. Various estimates range from
2,500 to 10,000, depending on how many of the smaller features are
included. They are scattered through many regions of the Park, but most
are clustered in a few areas called geyser basins, where there are
continuous displays of intense thermal activity. (See frontispiece.) The
"steam" that can be seen in thermal areas is actually fog or water
droplets condensed from steam; so the appearance of individual geyser
basins depends largely on air temperature and humidity. On a warm, dry
summer day, for example, the activity may seem very weak (fig. 44),
except where individual geysers are erupting. On cold or very humid
days, however, "steam" plumes are seen rising from every quarter.

NORRIS GEYSER BASIN, as viewed northward from the Norris
Museum. This is one of the most active thermal areas in Yellowstone, but
the photograph was taken on a warm dry summer day when little hot-water
and steam activity was visible from a distance. Clouds of water
droplets (the visible "steam" in thermal areas) normally form only when
the air is cool and (or) moist. The floor of the basin is covered by a
nearly solid layer of hot-spring deposits. (Fig. 44)

How a thermal system operates

An essential ingredient for thermal activity is heat.
A body of buried molten rock, such as the one that produced volcanic
eruptions in Yellowstone as late as 60,000 to 75,000 years ago, takes a
long time to cool. During cooling, tremendous quantities of heat are
transmitted by conduction into the solid rocks surrounding the magma
chamber (fig. 45). Eventually the whole region becomes much hotter than
non-volcanic areas (fig. 46). Normally, rock temperatures increase
about 1°F per 100 feet of depth in the earth's crust, but in the
thermally active areas of Yellowstone the rate of temperature increase
is much greater. The amount of heat given off by the Upper Geyser Basin,
for example, is 800 times the amount given off by normal (nonthermal)
areas of the same size. This excess heat is enough to melt 1-1/2 tons
of ice per second! And, contrary to popular opinion, the underground
temperatures have not cooled measurably in the 100 years that records
have been kept on the thermal activity in the Park. In fact, geologic
studies indicate that very high heat flows have continued for at least
the past 40,000 years.

HEAT FLOW AND SURFACE WATER. Diagram showing a thermal
system, according to the explanation that water of surface origin
circulates and is heated at great depths. (Based on information
supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H.
Truesdell.) (Fig. 45)

INFRARED IMAGE of a part of Upper Geyser Basin. Infrared
instruments, sensitive to heat, are able to detect "hot" spots in the
landscape. Note especially the sharp "image" of Old Faithful. (Image
courtesy of National Aeronautics and Space Administration.) (Fig
46)

A second, equally essential ingredient for thermal
activity is water. Many thousands of gallons are discharged by the hot
springs and geysers in Yellowstone every minutewhere does all
this water come from? Studies show that nearly all the water originates
above ground as rain or snow (meteoric water; fig. 45), and that very
little comes from the underlying magma (magmatic water).

The mechanism for heating the water, on the other
hand, is a matter of some uncertainty. Until a few years ago the
heating was assumed to occur near the ground surface
and to be caused by hot magmatic gases (mostly steam) rising from the
underlying magma chamber. Deep wells drilled recently in many thermal
areas throughout the world (including research drill holes in
Yellowstone), however, suggest a better explanation. According to this
explanation, the surface water enters underground passages (fractures
and faults) and circulates to great depthsas much as 5,000-10,000
feet in some areas (fig. 45)there to become heated far
above its surface boiling point. Research drill holes in Yellowstone,
for example, have demonstrated that water of surface origin exists at
all depths at least to the maximum drilled (1,088 feet), and that the
water reaches temperatures up to at least 465°F. The increase in
temperature with depth causes a corresponding decrease in the weight
(density) of the water. Because of this, the hot, "lighter," water
begins to rise again toward the ground surface, pushed upward by the
colder, "heavier," near-surface water which sinks to keep the water
channels filled. Thus is set into motion a giant convection
current which operates continuously to supply very hot water to the
thermal areas (fig. 45). Just how deep the waters circulate in
Yellowstone no one really knows; as a guess, the depth probably is at
least 1 or 2 miles.

The effect of pressure on the boiling temperature of
water also plays a vital role in thermal activity. In a body of water,
the pressure at the surface is that exerted by the weight of air above
it (atmospheric pressure). Water under these conditions boils at
212°F at sea level and at about 199°F at the elevation of most of
the geyser basins in Yellowstone. However, water at depth not only is
subjected to atmospheric pressure but also bears the added weight of the
overlying water. Under such additional pressures, water boils only when
the temperature is raised above its surface boiling point. In a well 100
feet deep at sea level, for example, the water at the bottom would have
to be heated to 288°F before it will boil. Thus it follows that in the
underground "workings" of hot springs or geysers, (1) The deepest water
is subjected to the greatest pressures, and (2) these deeper waters (in
Yellowstone) must be heated well above 199°F before they can actually
begin to boil. By this same reasoning but in reverse, if the pressure is
released, which happens as the water rises toward the ground surface, the
"hotter-than-boiling" water will then begin to boil. The boiling will be
rather quiet if the pressure is released gradually, as in most hot
springs. But if the pressure is released suddenly, boiling may become so
violent that much of the water flashes explosively into steam, expanding
to several hundred times its normal volume. This expansion provides the
necessary energy for geyser eruptions.

Hot-spring deposits and algae

Nearly all geysers and many hot springs build mounds
or terraces of mineral deposits; some are so unusual in form that
descriptive names have been given to them, such as Castle Geyser (fig.
47). These deposits are generally made
up of many very thin layers of rock. Each layer
represents a crust or film of rock-forming mineral which was originally
dissolved in hot water as it flowed through the underground rocks, and
which was then precipitated as the water spread out over the surrounding
ground surface.

MOUND OF SINTER at Castle Geyser, Upper Geyser Basin.
Lower part of mound has well-defined layers probably deposited by normal
hot springs. The upper, irregular part resulted from the vigorous
eruptions characteristic of geysers and marks a change in the local
hot-spring activity. (Fig. 47)

In all major thermal areas of the Park, with the
exception of Mammoth Hot Springs, most of the material being deposited
is sinter (the kind found around geysers is popularly called
geyserite). Its chief constituent is silica (the same as in
quartz and in ordinary window glass). At Mammoth, the deposit is
travertine (fig. 48), which consists almost entirely of calcium
carbonate. The material deposited at any given place commonly reflects
the predominant kind of rock through which the hot water has passed
during its underground travels. At Mammoth Hot Springs the water passes
through thick beds of limestone (which is calcium carbonate), but in
other areas the main rock type through which the water percolates is
rhyolite, a rock rich in silica.

Through centuries of intense activity, layers of
sinter have built up on the floors of the geyser basins (fig. 44); these
deposits are generally less than 10 feet thick. In one drill hole at
Mammoth, deposits of travertine extend to a depth of 250 feet. Dead
trees and other kinds of vegetation whose life processes have been
choked off by the heat, water, and precipitated minerals of hot-spring
activity are a common sight in many places (fig. 51).

Both travertine and sinter are white to gray. Around
active hot springs, however, the terraces that are constantly under
water may be brightly colored (figs. 43 and 49) because they are coated
by microscopic plants called algae. These organisms, which thrive
in hot water at temperatures up to about 170°F, are green, yellow, and
brown. Oxides of iron and manganese also contribute to the coloring in
some parts of the thermal areas. The delicate blue color of many pools,
however, results from the reflection of light off the pool walls and
back through the deep clear water (fig. 43). Other pools are yellow
because they contain sulfur, or are green from the combined influence of
yellow sulfur and "blue" water.

ALGAL-COLORED TERRACES lining the west bank of the
Firehole River at Midway Geyser Basin. Algae are microscopic plants that
grow profusely on rocks covered by hot water at temperatures up to about
170°F. (Fig. 49)

Hot springs and geysers

Hot springs occur where the rising hot waters of a
thermal system issue from the ground-level openings of the feeder
conduits (fig. 45). By far the greatest numbers discharge water and
steam in a relatively steady noneruptive manner, although they vary
considerably in individual behavior. Depending upon pressure, water
temperature, rate of upflow, heat supply, and arrangement and size of
underground passages, some hot springs boil violently and emit dense
clouds of vapor whereas in others the water quietly wells up with little
agitation from escaping steam. In some hot springs, however, the
underground channels are too narrow or the upflow of very hot water and
steam is too great to permit a steady discharge; periodic eruptions then
result. These special kinds of springs are called "geysers" (from the
Icelandic word geysir, meaning to "gush" or "rage"). At least
200 geysers, of which about 60 play to a height of 10 feet or more,
occur in Yellowstone National Park; this is more than in any other
region of the world.

How does a geyser work? We cannot, of course, observe
the inner plumbing of a geyser, except for that part which is seen by
looking into its uppermost "well." Deeper levels directly below the
"well" can be probed by scientific instruments to some extent, and
research drilling in some parts of the geyser basins also provides much
useful information. The available information suggests that the plumbing
system of a geyser (1) lies close to the ground surface, generally no
deeper than a few hundred feet; (2) consists of a tube, commonly nearly
vertical, that connects to chambers, side channels, or layers of porous
rock, where substantial amounts of water can be stored; and (3) connects
downward through the central tube and side channels to narrow conduits
that rise from the deepwater source of the main thermal system.

Considering a geyser system as described above and
applying what is known about the behavior of water and steam, we can
understand what causes a natural thermal eruption. Figure 50 shows
diagrammatically the succession of events believed to occur during the
typical eruptive cycle of a geyser such as Old Faithful.

A GEYSER IN ACTION. Photographs of successive stages
in the eruption of Old Faithful illustrate what probably happens during
a natural geyser eruption. The underground plumbing is diagrammatic and
does not reflect any specific knowledge of Old Faithful's system.
Direction of flow of water is shown by arrows. (Based on information
supplied by D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H.
Truesdell.)

Stage 1 (Recovery or recharge stage). After an eruption, the partly
emptied geyser tubes and chambers fill again with water. Hot water
enters through a feeder conduit from below, and cooler water percolates
in from side channels nearer the surface. Steam bubbles (with some other
gases such as carbon dioxide and hydrogen sulfide) start to form in
upflowing currents, as a decrease in pressure causes a corresponding
decrease in boiling temperature. At first the bubbles condense in the
cooler, near-surface water that is not yet at boiling temperature, but
eventually all water is heated enough that the bubbles will no longer
condense or "dissolve."

Stage 2 (Preliminary eruption stage). As the rising gas bubbles grow in
size and number, they tend to clog certain parts of the geyser tube,
perhaps at some narrow or constricted point such as at A. When this
happens, the expanding steam abruptly forces its way upward through the
system and causes some of the water to discharge from the surface vent
in preliminary spurts. The deeper part of the system, however, is not
yet quite hot enough for "triggering."

Stage 3 (Full eruption stage). Finally, a preliminary spurt "unloads"
enough water (with resulting reduction in pressure) to start a chain
reaction deeper in the system. Larger amounts of water in the side
chambers and pore spaces begin to flash into steam, and the geyser
rapidly surges into full eruption.

Stage 4 (Steam stage). When most of the extra energy is spent, and the
geyser tubes and chambers are nearly empty, the eruption ceases. Some
water remains in local pockets and pore spaces, continuing to make steam
for a short while. Thereafter the system begins to fill again, and the
eruptive cycle starts anew. (click on image for an enlargement in a
new window) (Fig. 50)

No two geysers have the same size, shape, and
arrangement of tubes and chambers. Also, some geysers, such as Great Fountain,
have large surface pools not present in cone-type geysers such as Old
Faithful. Hence, each geyser behaves differently from all others in
frequency of eruption, length of individual eruptions, and amount of
water discharged. Geysers may also vary in their own behavior as their
plumbing features change through the years. The great amount of energy
that builds up in some of them from time to time creates enough
explosive force to shatter parts of the plumbing system, thereby
causing a change in their eruptive behavior. In fact, some geyser
eruptions have been so violent that large chunks of rock have been
exploded out of the ground and scattered around the surrounding area
(fig. 51). With time, the precipitation of minerals may partly seal a
tube or chamber, gradually altering the eruptive mechanism.

SEISMIC GEYSER, showing rock rubble blown out during an
explosive thermal eruption. Note the trees that have been killed by the
heat and eruptive activity. According to George D. Marler of the
National Park Service, this geyser developed from cracks caused by the
Hebgen Lake Earthquake of August 17, 1959. (Fig. 51)

Despite all the variable factors involved in geyser
eruptions, and all the changes that can take place from time to time to
alter the pattern of those eruptions, several of the Yellowstone geysers
function regularly, day after day, week after week, and year after year.
Within this group of regulars is the most famous feature of allOld
Faithfulwhich has not missed an eruption in all the many
decades that it has been under close observation (fig. 52). We can only
conclude that nature has provided this incredible geyser with a stable
plumbing system that is just right to trigger delightfully graceful
eruptions at close-enough time intervals to suit the convenience of all
Park visitors.

OLD FAITHFUL IN FULL ERUPTION. The interval between
eruptions averages about 65 minutes, but it varies from 33 to 96
minutes. The time lapse between eruptions can be predicted rather
closely, mainly on the basis of the length of time involved in the
previous eruption. If an eruption lasted 4 minutes, for example, this
means that a certain amount of water emptied from the geyser's chambers
and that a certain length of time will be necessary to recharge the
system for the next eruption. But if the previous eruption lasted only 3
minutes, less time will be needed for recharge, and the next eruption
will occur sooner. (The above discussion is based primarily on many
years of observation and study of Old Faithful by George D. Marler and
other observers of the National Park Service; photograph courtesy of
Sgt. James E. Jensen, U.S. Air Force.) (Fig. 52)

Mudpots

Mudpots are among the most fascinating and
interesting of the Yellowstone thermal features. They are also a type of
hot spring, but one for which water is in short supply. Whatever water
is available becomes thoroughly mixed with clay and other fine
undissolved mineral matter. The mud is generally gray, black, white, or
cream colored, but some is tinted pale pink and red by iron compounds
(fig. 43); hence, the picturesque term "paint pots" is commonly
used.

Mudpots form in places where the upflowing thermal
fluids have chemically decomposed the surface rocks to form
clay. Such small amounts of water are involved,
however, that the surface discharge is not great enough to flush the
clay out of the spring. Caldrons of mud of all consistencies result,
from the very thin soupy material in many mudpots to the almost
hard-baked material in the less active features. Some mudpots expel
pellets of very thick viscous mud which build up circular cones or
mounds; this type is commonly called a "mud volcano" (fig. 53).

MUD VOLCANO near Pocket Basin in the Lower Geyser Basin.
The mud is formed by chemical decomposition of the rocks chiefly by the
action of carbon dioxide and sulfuric acid. The splatter, 5-6 feet high,
is caused by the escaping gases. (Fig. 53)

Mudpot activity differs from season to season
throughout the year because of the varying amounts of rain and snow that
fall upon the surface to further moisten the mud. Accordingly, mudpots
are commonly drier in late summer and early fall than they are from
winter through early summer.

Fumaroles

Fumaroles (from the Latin word fumus, meaning
"smoke") are those features that discharge only steam and other gases
such as carbon dioxide and hydrogen sulfide; hence, they are commonly
called "steam vents." Usually these features are perched on a hillside
or other high ground above the level of the flowing springs. In many
fumaroles, however, water can be heard boiling violently at some lower,
unseen level.

Thermal explosions

A few features present in the Yellowstone thermal
areas display evidence that extremely violent thermal explosions
occurred in the past, particularly during Pinedale Glaciation, about
15,000 years ago. Such explosion features, of which Pocket Basin in
Lower Geyser Basin is a good example, appear as craterlike depressions a
few tens of feet to as much as 5,000 feet across surrounded by rims of
rock fragments that were blown out of the craters. The underground
mechanism causing the explosions was similar to that of geysers, but in these
special cases the energy remained bottled-up until a very critical
explosive stage was reached.

The best explanation for Pocket Basin and related
features is that the ground above the sites of the explosions was weighted
down by the water of small lakes which had formed in melted-out pockets
of glacial ice. Such localized melting of the glaciers would occur where
the ice was in direct contact
with underlying thermal features. A rapid draining of
the lake waters would then produce a sudden release of pressure over the
hot area, resulting in an unusually violent thermal eruption.

Faulting and its control of thermal activity

Most of the major thermal areas of Yellowstone are
related to the ring fracture zones of the Yellowstone caldera (fig. 22).
Many deep-seated faults and fractures in these zones are presumably
situated above the main source of heat of the thermal system. Thus, they
provide convenient avenues of travel for underground waters to circulate
to great depths, there to become heated and then rise to the earth's
surface (fig. 45). A few areas like Mammoth Hot Springs and Norris
Geyser Basin, on the other hand, are not within the ring fracture zones
of the caldera. In these areas, the thermal activity is commonly related
to other prominent zones of faulting which also afford readymade
channelways for the circulation of hot water and steam.